1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a frequency converter utilized in a wind turbine generator.
2. Description of the Related Art
Power grids are the part of a power supply system and have the components necessary to transfer the electric power generated by power generating units across long distances to points of power consumption. Most power grids installed today carry energy as an alternating voltage and current. It is worth highlighting that the number of power grids carrying energy as continuous voltage and current (DC) is increasing due to the advantages they offer in terms of energy efficiency in long-distance grids. This has been possible due to the progress in conversion systems based on power electronics, which allow interconnecting the two types of grids, alternating power grids and continuous power grids, by using HVDC (High Voltage Direct Current) and HVAC (High Voltage Alternating Current) conversion structures.
Similarly, the progress experienced in power electronics is favoring a change in a new direction towards a distributed generation structure and away from the basic generation structure used until now, which has been based mainly on large thermal, hydraulic or nuclear power stations. One of the key elements in the growing structure of distributed generation is wind power, which in the last decade has experienced large growth by way of new installations of wind power generators. Wind power generation relies on power electronics, since most of the generators used to convert the mechanical power of wind into electric power injected into the grid are controlled by conversion structures based on power electronics, especially components known as frequency converters.
Frequency converters are controlled from control units that perform control based mainly on information captured using current and voltage transducers, and by running control algorithms to control the power flow between two electrical systems. There are different kinds of electrical systems, such as power grids or electrical machines, and the power flow can be bidirectional. For example, if the energy is consumed from the power grid in order to be transformed into mechanical energy in the shaft of an electric machine, the application will correspond to a motor application (for example, pumping or ventilation applications). In contrast, if the energy is extracted from an electric machine and injected into the power grid, the application will correspond to a generation application (for example, wind generation applications, where the primary power source is the wind, which turns the shaft of the electric machine).
Alternating power grids consist mainly of cables (the physical medium through which energy flows) and voltage transformers (components that allow adapting voltage levels between different connection points). Both components, cables and transformers, are components having a mainly inductive nature, and therefore, provide inductive impedance to the alternating current circulating through them. Depending on the characteristics of each power grid, the existing inductive impedance will vary, the length of the cables in the grid being an important parameter to consider when quantifying the value of inductive impedance (the greater the length, the greater the grid inductance and therefore the greater the inductive impedance). The existence of high inductive impedance in a power grid will mean a greater loss in its transmission capacity. This phenomenon is due to the voltage drop that occurs in the inductive impedance of the cable when current circulates through it, and it can become important in certain cases that combine factors such as long lengths of cable and high power consumption (high circulation of current through the power grid).
In the related art, there exist some solutions to the aforementioned problem of a loss of transmission capacity in power grids having a high inductive impedance. One of the commonly applied solutions relies on the compensation of the highly inductive power grids by inserting capacitive components (condensers) in series. This functions to compensate for the inductive impedance of the power grid itself by inserting series capacitive impedance, resulting in the reduction of the total equivalent impedance. This technique minimizes the problem of voltage drops in the power grid and therefore contributes to maintaining its capacity for power transmission.
Inserting series capacitors in highly inductive power grids is effective when solving the problem of the loss of transmission capacity of a grid, but in turn, results in problematic effects when considered from the point of view of the stability of the compensated power grid. Specifically, the insertion of series capacitors within an inductive grid results in the equivalent circuit of that grid having a natural resonant frequency according to the formula described by:
                              f          R                =                              f            0                    ⁢                                                    X                C                                            X                L                                                                        Equation        ⁢                                  ⁢                  (          1          )                    Wherein:fR—Natural resonant frequency of the compensated gridf0—Base frequency of the power gridXc—Capacitive impedance of the series condenser inserted in the power gridXL—Inductive impedance of the power grid
FIG. 2 shows a single-line wire diagram for a power grid compensated with series capacitors. The different components making up the power grid are: the central power generation unit 13 shown in the figure as a wind farm; the equivalent inductance 14 of the transmission lines or power grid cables; the condensers introduced in series in the power grid to compensate for the equivalent inductance of the power grid; and the collectors 15 present in the power grid that join the transmission lines from different points.
Depending on the degree of compensation applied to the power grid (percentage of capacitive impedance as series capacitors with respect to the inductive impedance of the power grid itself), the resulting value of the resonant frequency of the power grid will vary. The ratio of capacitive and inductive impedances commonly applied in power grid compensation usually results in resonant frequency values lower than the grid's base frequency. The technical literature uses the term SSI (Sub Synchronous Interactions) to describe the condition of a power grid having these features.
Grids with subsynchronous resonance are potentially dangerous grids for integrating generation components that are based on generation turbines with rotary shafts having a low-frequency mechanic oscillation. This is the case of synchronous generators with long shafts (typical example of generation plants) in which the distribution of mass along the shaft that in turn rotates by the action of a primary torque source (steam, water, etc.), commonly exhibit mechanical oscillation modes with frequencies of less than the base frequency of the power grid to which they are connected. In the event that the grid connected to a generator with the features mentioned above is compensated with a specific value of series capacitors that make the natural subsynchronous resonant frequency coincide with the oscillation frequency of the mechanical shaft, negative effects may be inducted on the shaft, since the amplitude of the mechanical oscillation of the shaft could be amplified with a negative damping (that is, an oscillation of increasing amplitude over time). This effect could result in a failure of the generator shaft. This case corresponds to a problem specific to the natural interaction between two parts of a power system, the power grid adjusted with capacitors and the generator, where the mechanical mass of a synchronous generator resonates with the subsynchronous frequency of the grid equivalent impedance of the power grid. This phenomenon is known in the technical literature as SSR (Subsynchronous Resonance).
In addition to the possibility of the natural resonant frequency of a compensated power grid coinciding with the natural mechanical resonant frequency of generators injecting power into that grid, the growing presence of frequency converters connected to the grid adds a new aspect to be considered from the point of view of grid stability. This is the interaction of frequency converter controls with compensated grids, a phenomenon that can cause the loss of control of the flow of energy through the converter, which may destabilize the power grid itself. This phenomenon is known in the literature as SSCI (Subsynchronous Control Instability).
The SSCI phenomenon occurs when control of the frequency converters connected to compensated grids having series capacitors makes the converters behave as electrical systems whose equivalent resistance acquires negative values within a frequency range of less than the grid base frequency. The SSCI phenomenon may have similar effects to those of the SSR phenomenon, but to achieve this there must be a high number of frequency converters connected to compensated grids. The increasing use of frequency converters connected to the grid, together with the existence of power grids compensated with series capacitors, have made this potentially hazardous scenario become a reality, for which a solution is desired and which forms a basis for the invention disclosed herein.